Mobility handling in a communication network

ABSTRACT

A method and apparatus for handling mobility for a first host with a MPTCP connection to a second host. The first host is identified by a master address that does not change for the duration of the connection, and a plurality of locators. Each locator is associated with a path used in the MPTCP connection. The first host then determines that it has moved, and so obtains new locators. It then sends a first message to a Location Server (LS). The LS maintains an association between the master address and the locators, and the first message includes the new locators. The first host then sends a second message to the second host, the second message including the new locators. The first host can then receive packets sent from the second host sent using the MPTCP connection using the new locators.

TECHNICAL FIELD

The invention relates to the field of mobility handling in acommunication network, and in particular to handling mobility for aMultipath Transmission Control Protocol connection.

BACKGROUND

The Transmission Control Protocol (TCP) is a core protocol used incommunication networks. It allows data exchange between an applicationat one host and an application at another host. A limitation of TCP isthat although hosts are often connected to one another by multiplepaths, TCP restricts communications to a single path for each transportconnection. In order to more efficiently use resources within a network,Multipath TCP (MPTCP) is under development, as described in Ford et al,“TCP Extension for Multipath Operation with Multiple Address,”draft-ford-mptcp-multiaddressed-03 (Experimental), March 2010 and Fordet al, “Architectural Guidelines for Multipath TCP Development”,draft-ietf-mptcp-architecture-00 (Informational), February 2010.

Using MPTCP, hosts that have access to multiple paths can use these fora given TCP connection to exchange data. The simultaneous use ofmultiple paths for a TCP/IP session improves resource usage within thenetwork, and therefore improves user experience through higherthroughput and improved resilience to network failure.

Increasingly, host mobility is commonly required by endpoints. Themovement of a host causes a reduction in TCP performance. There areseveral protocols which have been proposed for handling host mobilitymanagement, including Mobile IP (MIP), as described in Johnson et al,“Mobility Support in IPv6”, RFC 3775, June 2004, and Host IdentityProtocol, as described in Moskowitz et al, “Host Identity Protocol”, RFC5201, April 2008. Most IP protocols that do not provide for hostmobility management use an IP address to identify not only the host, butalso the topological location of the host. MIP on the other handseparates the host ID and Location using a Home Address (HoA) and a Careof Address (CoA). Host mobility problem is managed by mapping the CoA(which can change as the host moves) to the HoA (which is a permanentlyor semi-permanently assigned address for the host in its home network).Any applications used to communicate with the host use the HoA for datatransmission. In the host's home network, a home agent is introduced formaintaining the mapping between the CoA and the HoA for the host. When ahost roams in a foreign network, it registers with the home agent. Thehome agent captures all the packets sent to the host's HoA and forwardsthem to the host by tunnelling to the CoA. In this way, even though thehost moves and receives a new CoA, the HoA remains the same for theduration of a session and the host can communicate with other hosts.

In a similar manner to MIP, HIP maintains a separate host identity andlocation information for each mobile node (MN). The Host Identity (HI)name space is to identify the host identity, while the locationinformation, i.e. IP address is used for the purpose of routing. ARendezvous server (RVS) is provided to maintain a mapping between the HIname space and the current IP address for the MN. The host name, HI, andRVS address are maintained by a Domain Name System (DNS) Server. Whenanother host initiates a connection towards the MN, it first resolvesthe HI and RVS address, and then sends initialization packets to theRVS. The RVS is responsible for forwarding the packets to MN. If the MNmoves, it receives a new IP address and communicates with the RVS toensure that the RVS has the updated IP address.

MIP and HIP provide solutions for host mobility management, but haveshortcomings if a host wishes to use the MPTCP protocol. A problem withMIP is that only one HoA is introduced into the transport layer, whereasMPTCP requires multiple IP addresses to identify each path used for eachMPTCP sub-flow. As MIP only assigns one HoA, it cannot support MPTCP.

A problem with HIP is that although it can support several simultaneoustransport sessions, it can only do this for different flows. In otherwords, it could support a TCP connection between email applications fortwo hosts, and simultaneously support a further TCP connection betweenInternet applications for two hosts. However, it can not supportsplitting a TCP session between the email applications of two hosts touse multiple paths for the same TCP connection.

HIP and MIP can handle host mobility management, but they cannot handlehost mobility management using MPTCP.

SUMMARY

The inventors have realised the problem with host mobility managementusing MPTCP. According to a first aspect, there is provided a method ofhandling mobility for a first host with a MPTCP connection to a secondhost. The first host is identified by a master address that does notchange for the duration of the connection, and a plurality of locators.Each locator is associated with a path used in the MPTCP connection. Thefirst host then determines that it has moved, and so obtains newlocators. It then sends a first message to a Location Server (LS). TheLS maintains an association between the master address and the locators,and the first message includes the new locators. The first host thensends a second message to the second host, the second message includingthe new locators. The first host can then receive packets sent from thesecond host sent using the MPTCP connection using the new locators.

As an option, and prior to determining that the first host has moved,the first host resolves a main address of the second host. A pathmanager function at the first host intercepts a synchronisation packetdestined for the second host, which includes a token identifying theMPTCP connection and a source and destination address. The path managerfunction resolves locators associated with the second host for use inthe MPTCP connection. Paths to use during the MPTCP connection arenegotiated, and each path is associated with a locator belonging to thefirst host and a locator belonging to the second host. The sourceaddress in the synchronisation packet is replaced with locatorsbelonging to the first host and the destination address in thesynchronisation packet is replaced with the locators belonging to thesecond host. The synchronisation packet is then sent to the second host.

The method optionally comprises receiving a further message from thesecond host. The further message includes new locators for the secondhost as a result of the second host moving. Packets are then sent to thesecond host packets using the MPTCP connection using the new locators.

According to a second aspect, there is provided a host device for use ina communication network. The host device is identified by a masteraddress that does not change for the duration of a MPTCP connection witha second host device, and a plurality of locators. Each locator isassociated with a path used in the MPTCP connection. The host device isprovided with a processor for determining that the host has moved andthe locators are no longer valid. It is further provided with means forobtaining new locators belonging to the host. A first transmitter isprovided for sending a first message to a LS, the LS maintaining anassociation between the master address and the locators, the firstmessage including the new locators. A second transmitter is provided forsending a second message to the second host, the second messageincluding the new locators. A receiver is also provided for receivingfrom the second host packets sent using the MPTCP connection using thenew locators.

As an option, the host device is provided with an application forresolving a main address of the second host. A path manager function isprovided to intercept a synchronisation packet, the synchronisationpacket destined for the second host and including a token identifyingthe MPTCP connection and a source and destination address. The pathmanager function is also arranged to resolve locators associated withthe second host for use in the MPTCP connection. A multipath negotiatingfunction is provided to negotiate paths to use during the MPTCPconnection. An address translation function is provided for associating,with each path, a locator belonging to the host device and a locatorbelonging to the second host. The address translation function is alsoarranged to replace the source address in the synchronisation packet andfuture packets with locators belonging to the host device and thedestination address in the synchronisation packet with the locatorsbelonging to the second host. A third transmitter is provided forsending the synchronisation packet to the second host.

According to a third aspect, there is provided a LS for use in acommunication network. The LS is provided with a memory for storing, foreach of a plurality of host nodes, a master address and a plurality oflocators. A receiver is provided for receiving from a first host, priorto establishing a MPTCP connection with a second host, a requestcontaining the master address of the second host. A processor determinesthe locators associated with the master address for the second host. Atransmitter is provided for sending to the first host the locatorsbelonging to the second host.

As an option, the receiver is further arranged to receive from the oneof the first and second hosts a plurality of new locators. In this case,the processor is arranged to replace the existing locators stored in thememory with the new locators.

According to a fourth aspect, there is provided a host device for use ina communication network, the host device being able to communicate witha further host device, the further host device being identified by amaster address that does not change for the duration of a MPTCPconnection with the host device, and a plurality of locators, eachlocator associated with a path used in the MPTCP. The host device isprovided with a memory for storing a plurality of locators, theplurality of locators being associated with the further host. A receiveris provided for receiving from the further host device a message, themessage including new locators associated with the further host. Aprocessor is also provided for replacing the association in the memorybetween the plurality of locators and the further host device with anassociation in the memory between the new locators and the further host.

According to a fifth aspect, there is provided a computer program thatcomprises computer readable code means which, when run on a host device,causes the host device to perform a method as described above in thefirst aspect.

According to a sixth aspect, there is provided a computer program thatcomprises computer readable code means which, when run on a LS, causesthe LS to behave as a LS as described above in third aspect.

According to a seventh aspect, there is provided a computer programproduct comprising a computer readable medium and a computer program asdescribed above in the fifth and sixth aspects, wherein the computerprogram is stored on the computer readable medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically in a block diagram a networkarchitecture according to an embodiment of the invention;

FIG. 2 is a signalling diagram showing signalling required to set up aMPTCP connection according to an embodiment of the invention;

FIG. 3 is a signalling diagram showing signalling when one host movesduring an ongoing MPTCP connection according to an embodiment of theinvention;

FIG. 4 is a signalling diagram showing signalling when both hosts moveduring an ongoing MPTCP connection according to an embodiment of theinvention;

FIG. 5 illustrates schematically in a block diagram a host deviceaccording to an embodiment of the invention;

FIG. 6 illustrates schematically in a block diagram a Location Serveraccording to an embodiment of the invention; and

FIG. 7 illustrates schematically in a block diagram a host deviceaccording to a further embodiment of the invention.

DETAILED DESCRIPTION

For each host, a main identity (MA) is assigned along with severallocators. The MA is a globally unique IP address that identifies theMPTCP host. The MA is permanent or semi-permanently assigned (in otherwords, it will not change for the duration of a MPTCP connection). TheMA is not used for routing in network transmission, and only used by theapplication layer and the transport layer. Locators, on the other hand,are changeable IP addresses obtained from the network in which the hostis located. Locators are routable for use in forwarding packets. Theprovision of several locators allows MPTCP to be used. Note that it ispossible for MPTCP to be used where only one host has multiple locators,and the other host uses only a single locator.

For each MPTCP host, a mapping between the MA and the locators ismaintained by a Location Server (LS). The mapping between host domainname, MA and LS address is stored in the DNS. Host mobility managementrelies on dynamic mapping of the MA and the locators.

Referring to FIG. 1 herein, there is illustrated a network architecture.A first host 1 is provided that has one or more applications 2 that mayrequire the use of MPTCP. A Multi-Path Scheduler (MPS) 3 and a PathManager (PM) 4 are also provided at the first host 1. The applications 2can communicate with the DNS 5. The PM 4 comprises a location client 6,which can communicate with the Locations Server 7. The PM 4 alsoincludes a multipath negotiator 8 for negotiating and controlling MPTCPwith another host, and address translation function 9 and a movementmanager 10.

For completeness, a second host 11 is shown that is also provided withone or more applications 12, a MPS 13, and a PM 14 that comprises anaddress translation function 15, a multipath negotiator 16, a locationclient 17 and a movement manager 18.

When the first host 1 wishes to creates a MPTCP connection towardsmobile MPTCP second host 11, it first resolves the second host's 11 MAaddress and LS address using the DNS 5. The PM 4 is responsible forquerying the mobile host's 11 locators from the LS 7. Multiple paths,identified by a path index, are negotiated by the MP negotiators 8, 16of the hosts 1, 11 and notified to the MPS 3 by the PM 4. The MPS 3creates multiple sub flows via the address translation function 9according to the negotiated paths. This allows several TCP sub-flows tobe exchanged between the address translation functions 9, 15 and passedto the MPS 3, 13 of each host 1, 11. Each MPS 3, 13 can then pass dataobtained from the TCP sub-flows to the applications 2, 12 involved inthe TCP communication.

In the transport layer, the MA of a mobile host 11 is used as the sourceand destination addresses and a 6-tuple (sMA, dMA, Protocol, src_port,dst_port, path_index) is used to describe a sub-flow (see Arkko and vanBeijum, “Failure Detection and Locator Pair Exploration Protocol forIPv6 Multihoming”, RFC 5534, June 2009). For outgoing packets, beforethe packets are delivered to the IP layer, the PM 4 rewrites the (sMA,dMA) to (sLocator, dLocator) according to the path index. In the reversedirection, packets are rewritten from (sLocator, dLocator) to (sMA, dMA,path_index).

The PM 4, 14 is also responsible for host mobility management, and sothe movement manager 10, 18 of each PM 4, 14 ensures that a host 1, 11updates its locators to the LS 7 in the event that it moves and obtainsnew locators. The movement manager 10, 18 notifications from anoperating system when a configuration between interfaces changes. Themovement manager 10, 18 must then decide whether or not the notificationis relevant to movement or not. If so, then the movement manager 10, 18informs notifies other modules about the changes in interfaceconfiguration. An example of such a change is a change in IP address.Another example is notification of loss of transmission media (e.g. nowireless coverage, network cable unplugged etc.). When the MP negotiator8, 16 receives this information from the movement manager 10, 18, itinstructs the location client 6, 17 to perform an update and also startsnegotiating new locators with the other host. When a locator haschanged, it is necessary to perform update in the mapping table betweenthe MA and new locator(s) in order to provide the correct translationfor subsequent packets between the hosts (in both directions).

FIG. 2 herein illustrates the steps required to set up a MPTCPconnection. The following numbering corresponds to that shown in FIG. 2.

S1. When an application 2 running at the first host 1 wishes toestablish a MPTCP connection with the second host 11, it resolves thesecond host's MA using a DNS query and uses the MA as the destinationaddress for the MPTCP connection. This information is sent to the MPS 3.

S2. The MPS 3 uses a default path to establish a single TCP connectionwith the second host 11 by sending a synchronize packet (SYN) toinitiate a three-way handshake with the second host 11. The source anddestination addresses used in the SYN are the MA of the first host 1 andthe second host 11. The SYN contains a token to identify the MPTCPconnection in a MPC option. The token and Multipath Capable (MPC) optionare described in detail in Ford et al, “TCP Extension for MultipathOperation with Multiple Address,” draft-ford-mptcp-multiaddressed-03(Experimental), March 2010. In summary, the MPC option declares that thefirst host is capable of MPTCP and wishes to use MPTCP for thisconnection. The token is a locally unique token used to identity theconnection, and can be used to add sub-flows to the connection. Notethat the token is unique for the first host 1. The MTCP operations aresignalled using TCP header fields.

S3-S4. The PM 4 intercepts the SYN packet before it is sent to the IPlayer and on to the second host 11. The PM 4 first checks to determineif it has already recorded this particular MPTCP connection using thetoken in the MPC option, and if there is no existing record for this theconnection the PM 4 records it. The PM then resolves the locators of thesecond host 11 using the LS 7 and, using the locators, initiates amultipath negotiation process.

S5-S8. The PM 4 of the first host 1 initiates a four-way handshake withthe PM 14 of the second host 11 for exchanging the locators used tobuild multiple paths for the MPTCP connection with the second host 11.

S9. Once the path negotiation process is complete, the availablemultiple paths are announced to the MPS 3, 13 of each of the first host1 and the second host 11. A path is simply a sequence of links between asender and a receiver (the hosts in this case) and, in this context, isdenoted by a locator pair of locators for the first host 1 and thesecond host 11.

S10. The PM 4 maps the locators to the MA of the second host 11 andrewrites the source and destination address of the SYN packets from theMA to locators according to the path index. The PM 4 then sends the SYNto the IP layer for forwarding to the second host 11.

S11. The PM 14 of the second host 11 intercepts the SYN packet from thefirst host 1 and rewrites the source and destination address using theMAs of the first host 1 and the second host 11. The SYN is thenforwarded to the MPS for further handling.

S12-S17. The second host 11 sends back the SYN and ACK to accept theMPTCP request of the first host 1, and it send ACKs to finish thethree-way handshake Once this has been completed, the hosts 1, 11 canexchange data using the MCTCP connection

Turning now to FIG. 3, the steps are illustrated according to anembodiment of the invention in which the first host changes location.The following numbering corresponds to the numbering of FIG. 3.

S18. The MCTCP connection is established between the first host 1 andthe second host 11.

S19. The PM 4 of the first host 1 determines that the first host 1 hasmoved. One way of making such a determination is for an operating systemto use an Application Programming Interface (API) for notifying the PM 4of the changes.

S20. The PM 4 then obtains new locators, although the MA of the firsthost 1 remains unchanged.

S21-S22. After obtaining the new locators, the PM 4 sends a message tothe LS 7 to update the locators. In step S22, the LS 7 acknowledgesreceipt of the new locators. New locators may be obtained either bymovement detection along with the notification about the change ininterface configuration, or by using the change in interfaceconfiguration to trigger the PM 4 to obtain the locators.

S23-S24. The PM 4 sends the new locators to the second host 11. Thesecond host 11, in step S24, acknowledges that it has received the newlocators.

S25. The PM 4 of the first host 11 and the PM 14 of the second host 11notify the respective MPS 3, 13 if any links between them areunavailable.

S26-S28. All of the packets sent to the first host 1 now use the firsthost's 1 new locators, allowing the MPTCP connection to be maintainedusing the new locators even though the first host has moved.

Referring now to FIG. 4, there is illustrated the signalling accordingto an embodiment of the invention in which both the first host 1 and thesecond host 11 move and require new locators. The following numberingcorresponds to the numbering of FIG. 4.

S29. While data is exchanged using the MPTCP connection between thefirst host 1 and the second host 11, the PMs 4, 14 of the first host 1and the second host 11 monitoring any movements of the hosts.

S30-S33. The PMs 4, 14 determine that each of the first host 1 and thesecond host 11 have moved. Each PM 4, 14 sends a message to the LS 7 toupdate the LS 7 with the new locators.

S34-S35. The PMs 4, 14 of each of the first host 1 and the second host11 fail to notify their new locators to each other for the obsoletelocators they own, as each host moves simultaneously. Both hoststherefore change their locators simultaneously, and so both hosts needto query the location server for the locators for the other host.

S36-S37. The PM 4 of the first host 1 queries the LS 7 to obtain the newlocators of the second host 11. Similarly, the second host 11 mustobtain new locators for the first host 1.

S38-S39. After getting the new locators, the negotiation of multiplepaths between first host 1 and second host 11 is performed in steps S38and S39.

S40. The PMs 4, 14 of each of the first host 1 and the second host 11notify the respective MPS 3, 13 if some links between them areunavailable.

S41. The data packets sent using the MPTCP connection use the newlocators of the first host 1 and the second host 11.

Referring now to FIG. 5, there is illustrated the first host 1 by way ofexample. The host 1 is provided with a processor 19 which determineswhen the host 1 has moved. Once the host has moved, new locators arerequired, and so means 20 are provided for obtaining new locators. Afirst transmitter 21 is provided for sending a first message thatincludes the new locators to the LS 7, and a second transmitter 22 isprovided for sending a message including the new locators to the secondhost 11. A receiver 23 is also provided for packets sent from the secondhost 11 using the MPTCP connection.

As described above, the host is also provided with functions that may beimplemented using either hardware or software. These include theapplications 2 which, among other things, resolves a main address of thesecond host, the path manager function 4, the multipath negotiatingfunction 8 arranged to negotiate paths to use during the MultipathTransmission Control Protocol connection, the address translationfunction 9, the movement manager 10 and the MPS 3.

The first host 1 may also be provided with a third transmitter 24 forsending SYN to the second host 2. A computer readable medium in the formof a memory 25 may also be provided which may be used to store a programwhich, when executed by the processor, causes the first host 1 to behaveas described above and implement the functions necessary to supportmobility for a MPTCP connection.

Referring now to FIG. 6, there is illustrated a LS 7. The LS 7 isprovided with a computer readable medium in the from of a memory 27 forstoring the master address and a plurality of locators (for example, ina database 28) for each host served by the LS 7. A receiver 29 isprovided for receiving a request from the first host 1. The requestincludes the MA of the second host 11. A processor 30 is provided fordetermining the locators associated with the MA for the second host 11,and a transmitter 31 is provided for sending to the locators belongingto the second host 11 to the first host 1. The receiver 29 may also bearranged to receive, from the second host 11, a plurality of newlocators, and the processor 31 may be arranged to replace the existinglocators stored in the memory 27 with the new locators.

The memory 27 may also be used to store a program 32 which, whenexecuted by the processor 30, causes the LS 7 to behave as describedabove and implement the functions necessary to support mobility for aMPTCP connection.

Turning now to FIG. 7, there is illustrated a host device 11 thatreceives new locators as described above. The host device is providedwith a computer readable medium in the form of a memory 33. The memorycontains a database 34 or other data structure used to store anassociation between the host 1 and its locators. A receiver 35 receivesa message that contains the new locators, and a processor 35 updates thedatabase 34 to replace the previous locators associated with the host 1with the new locators.

The memory 33 may also be used to store a program 37 which, whenexecuted by the processor 36, causes the host 11 to behave as describedabove and implement the functions necessary to support mobility for aMPTCP connection.

The invention provides mobility handling for hosts when using a MPTCPconnection. An advantage of using an end-to-end protocol based on MPTCPis that it has great tolerance in the event of dealing with networkfaults or breakages. For example, if a path fails because of hostmovement, the MPS can temporarily send packets from other paths. Afurther advantage of the present invention is that the only intermediatenode required is the location server, and so it does not existingrouters and stations in the network do not require modification. Thismakes the invention easy to deploy in existing networks.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention as defined in theappended claims.

The following acronyms have been used in this description:

-   ACK Acknowledgement-   API Application Programming Interface-   CoA Care-of Address-   DNS Domain Name System-   HI Host Identity-   HIP Host Identity Protocol-   HoA Home Address-   LS Location Server-   MA Main Address-   MIP Mobile IP Protocol-   MN Mobile Node-   MPS Multipath Scheduler-   MPTCP Multi-Path TCP-   PM Path Manager-   RVS Rendezvous server-   SYN Synchronize packet

1. A method of handling mobility for a first host with a MultipathTransmission Control Protocol connection to a second host, the methodcomprising: at the first host, the first host being identified by amaster address that does not change for the duration of the connection,and a plurality of locators, each of the locators associated with a pathused in the Multipath Transmission Control Protocol connection,determining that the first host has moved; at the first host, obtainingnew locators; sending a first message to a Location Server, the LocationServer maintaining an association between the master address and thelocators, the first message including the new locators; sending a secondmessage to the second host, the second message including the newlocators; and receiving from the second host packets sent using theMultipath Transmission Control Protocol connection using the newlocators.
 2. The method according to claim 1, further comprising, priorto determining that the first host has moved, at the first host:resolving a main address of the second host; at a path manager functionat the first host, intercepting a synchronisation packet, thesynchronisation packet destined for the second host and including atoken identifying the Multipath Transmission Control Protocol connectionand a source and destination address; at the path manager function,resolving locators associated with the second host for use in theMultipath Transmission Control Protocol connection; negotiating paths touse during the Multipath Transmission Control Protocol connection, andassociating with each path a locator belonging to the first host and alocator belonging to the second host; replacing the source address inthe synchronisation packet with locators belonging to the first host andthe destination address in the synchronisation packet with the locatorsbelonging to the second host; and sending the synchronisation packet tothe second host.
 3. The method according to claim 1, further comprising:receiving from the second host a further message, the further messageincluding new locators for the second host as a result of the secondhost moving; and sending to the second host packets sent using theMultipath Transmission Control Protocol connection using the newlocators included in the further message.
 4. A host device for use in acommunication network, the host device being identified by a masteraddress that does not change for the duration of a MultipathTransmission Control Protocol connection with a second host device, anda plurality of locators, each of the locators associated with a pathused in the Multipath Transmission Control Protocol connection, the hostdevice comprising: a processor for determining that the host has movedand the locators are no longer valid; circuitry for obtaining newlocators belonging to the host; a first transmitter for sending a firstmessage to a Location Server, the Location Server maintaining anassociation between the master address and the locators, the firstmessage including the new locators; a second transmitter for sending asecond message to the second host, the second message including the newlocators; a receiver for receiving from the second host packets sentusing the Multipath Transmission Control Protocol connection using thenew locators.
 5. The host device according to claim 4, furthercomprising, an application for resolving a main address of the secondhost; a path manager function arranged to intercept a synchronisationpacket, the synchronisation packet destined for the second host andincluding a token identifying the Multipath Transmission ControlProtocol connection and a source and destination address; the pathmanager function being further arranged to resolve locators associatedwith the second host for use in the Multipath Transmission ControlProtocol connection; a multipath negotiating function arranged tonegotiate paths to use during the Multipath Transmission ControlProtocol connection, an address translation function for associatingwith each path a locator belonging to the host device and a locatorbelonging to the second host; the address translation function beingfurther arranged to replace the source address in the synchronisationpacket and future packets with locators belonging to the host device andthe destination address in the synchronisation packet with the locatorsbelonging to the second host; and a third transmitter for sending thesynchronisation packet to the second host.
 6. A Location Server for usein a communication network, the Location Server comprising: a memory forstoring, for each of a plurality of host nodes, a master address and aplurality of locators; a receiver for receiving from a first host, priorto establishing a Multipath Transmission Control Protocol connectionwith a second host, a request containing the master address of thesecond host; a processor for determining the locators associated withthe master address for the second host; and a transmitter for sending tothe first host the locators belonging to the second host.
 7. TheLocation Server according to claim 6, wherein: the receiver is furtherarranged to receive from the one of the first and second hosts aplurality of new locators; the processor being is further arranged toreplace the existing locators stored in the memory with the new locatorsreceived from the one of the first and second hosts.
 8. A host devicefor use in a communication network, the host device being able tocommunicate with a further host device, the further host device beingidentified by a master address that does not change for the duration ofa Multipath Transmission Control Protocol connection with the hostdevice, and a plurality of locators, each locator associated with a pathused in the Multipath Transmission Control Protocol, the host devicecomprising: a memory for storing a plurality of locators, the pluralityof locators being associated with the further host; a receiver forreceiving from the further host device a message, the message includingnew locators associated with the further host; a processor for replacingthe association in the memory between the plurality of locators and thefurther host device with an association in the memory between the newlocators and the further host.
 9. A computer program, comprisingcomputer readable code stored in a non-transitory computer readablemedium, the computer readable code, means which, when run on a hostdevice, causes the host device to perform the method as claimed inclaim
 1. 10. A computer program comprising computer readable code storedin a non-transitory computer readable medium, the computer readablecode, when run on a Location Server, causes the Location Server tobehave as a Location Server as claimed in claim
 6. 11. (canceled)